1. Field of the Invention
The invention relates generally to a process of forming contacts for an integrated circuit (IC) and more particularly to a process of forming self-aligned silicide contacts from deposited silicon.
2. Description of the Related Art
Silicides are compounds formed by a reaction of silicon with titanium, tungsten, cobalt, or other refractory metals. Silicides are formed at the surface of polysilicon gates and diffused source/drain regions to reduce the sheet resistance of the polysilicon gates and the diffused regions. A reduction in the sheet resistance is desirable because it minimizes voltage drops along interconnect lines.
Silicides can be formed from a self-aligned process. The term "self-aligned" signifies that the process does not employ additional steps for the purpose of aligning the silicides with respect to the locations of the gate, source and/or drain. The self-aligned process for forming silicides is illustrated in FIGS. 1A-1E.
FIG. 1A illustrates a semiconductor structure having a silicon substrate 10 on which a polysilicon gate 19 and diffused source and drain regions 21, 22 are formed. A silicon oxide (SiO.sub.2) layer 11 functions as a field oxide that isolates this semiconductor structure from others. FIG. 1B illustrates the semiconductor structure with oxide spacers 30 formed at the sidewalls of the polysilicon gate 19.
Next, a refractory metal 40 is deposited, and the semiconductor substrate 10 is heated. The heating causes a silicide reaction wherever the refractory metal 40 is in contact with silicon. Silicides 50, 51 and 52 are thereby formed. Everywhere else, the refractory metal 40 remains unreacted. The unreacted metal is selectively removed through the use of an etchant that does not attack the silicides 50, 51 and 52, the semiconductor substrate 10, or the SiO.sub.2 layer 11 (FIG. 1D).
A dielectric layer 60 is deposited onto the structure of FIG. 1D and contact holes 61, 62 are opened in the dielectric layer 60 down to the silicides 51, 52. Metal is then deposited into the contact holes to make contact with the silicides 51, 52 (FIG. 1E).
In forming the silicides 51, 52 by the above-described process, the silicon in the source and drain regions 21, 22 are consumed. If too much silicon is consumed from the source and drain regions 21, 22, the junctions formed in these regions will be destroyed. Therefore, the silicides 51, 52 are limited in thickness.
Furthermore, the above-described process is self-aligned only with respect to the silicides 51, 52. The metal contacts that are formed in the contact holes 61, 62 are not self-aligned. Therefore, additional alignment steps are necessary to assure that the contact holes 61, 62 are positioned correctly with respect to the locations of the gate, source, and drain.